1. Field of the Invention
This invention relates to a method of manufacturing microstructures by the anisotropic etching and bonding of substrates so as to manufacture an optical divider, an ink jet nozzle or a diaphragm variously by bonding silicon substrates different in the direction of crystallization and anisotropically etching those substrates properly using an etching solution which changes in etching characteristic according to the direction of crystallization.
2. Description of the Prior Art
Generally speaking, a silicon substrate is not only used typically to manufacture an electric element but also used widely to manufacture microstructure.
In the case where a semiconductor integrated circuit or a small mechanical structure is manufactured on a silicon substrate, silicon substrate bonding technique and anisotropical etching technique are used widely. The silicon substrate bonding technique is an art by means of which a number of silicon substrates are made into a sheet of substrate by bonding them after going through a properly pre-treating process. A process of making a silicon substrate hydrophillic, which is a pre-treating process, is performed by soaking it in a solution in which sulfuric acid and hydrogen peroxide are mixed in the ratio of 1 to 4.
The silicon substrates which have gone through the pretreating process are bonded under atmospheric pressure at the room temperature after mirror surfaces are made to face each other. At this time, these two substrates are heat-treated for about 1 hour in an oxidizing atmosphere usually at the temperature above 900.degree. C. to increase their bonding strength. When two substrates are bonded, those parts which are not bonded are caused by those particles interposed between the substrates, the flatness of the substrates or gas kept between the substrates, and so bonding must be conducted only in the same place to impose an effective check on it.
Bonding of substrates is finished if it is done in this way. As needs arise, substrates can be bonded after an oxidized film is formed on the surface of one substrate or after polycrystalline silicon is deposited thereon.
As the direction of silicon crystallization produces no effect on the bonding of substrates, silicon substrates crystallized either in the same direction or in a different direction can be bonded.
Examining those prior patent materials which made mention of such substrate bonding technique, the Japanese patent 61-182239 (A device for bonding of semiconductor substrates) refers to "a device designed to cause two substrates to achieve early bonding only at one point in order to prevent occurrence of a part which is not bonded due to gas kept between them when they are bonded" and the U.S. Pat. No. 3,332,137 (A method of insulating semiconductor material substrates) refers to "a method of insulating each range by etching bonded substrates formed of n type and p type layers in a vertical direction" and No. 3783218 (A method of bonding substrates electrostatically) refers to "a method of bonding substrates by applying electric field to lower the substrate bonding temperature.
In the European patent No. 0161740 (A method of manufacturing a semiconductor substrate), there are mentioned "technique of bonding substrates with n type and p type ranges manufactured vertically and technique of bonding substrates after manufacturing channels and structures thereon".
However, those techniques mentioned in the prior patent materials are useful only when substrates are bonded for manufacturing an element which functions electrically. No reference was made to bonding of substrates for manufacturing mechanical functioning structures such as a diaphragm, an optical divider and a nozzle. Reference was made only to isotropic etching of bonded silicon substrates.
Touching on a paper on bonded silicon substrates, technique of bonding the substrates crystallized in the same direction of &lt;100&gt;/&lt;100&gt; and &lt;111&gt;/&lt;111&gt; and bonding the substrates crystallized in a different direction of &lt;100&gt;/&lt;111&gt;, valuation of the physical properties of bonded interface and valuation of the properties of electric current-voltage by manufacturing a diode are mentioned in pages 533-536, 18th SSDM (1986), Ext, Abs. "Lattice configuration and electrical properties at the interface of direct bonded silicon" of K. Furukawa.
However, this paper limits the kinds of substrates to be bonded to &lt;100&gt; and &lt;111&gt;, making reference only to bonding of substrates for manufacturing an electrically functioning element.
Now, anisotropic silicon etching technique will be described. A peculiarity of wet etching varies with the crystallized direction of a silicon substrate and the concentration of an impurity, and so variously shaped structures can be manufactured if it is utilized. Solutions generally used for etching include a mixed solution of fluoric acid-nitric acid-acetic acid, a solution of EDP (ethylene diamine pyrocathecol) and a solution of KOH.
As to the mixed solution of fluoric acid-nitric acid-acetic acid, etching speed can be adjusted according to its ratio of composition. When it is in the ratio of specific composition (1-3-8), a p.sup.+ substrate is faster than a p.sup.- substrate by about 150 times in etching speed, and so only the p.sup.- range can be left by selectively removing only the p.sup.+ range. This solution is an isotropic etching solution which effects etching at the same speed for all crystallization directions in any composition. The solution of EDP changes in etching characteristic according to the concentration of an impurity and the direction of crystallization and scarcely etches an oxidized film, and so the oxidized film can be used as an etching protective film. Therefore, only the p.sup.- range can be removed selectively and the etching selection ratio of the surface can be made different according to the direction of crystallization.
The solution of KOH is an anisotropic etching solution which changes in etching characteristic according to the crystallization direction of silicon. In the direction of &lt;100&gt;, etching is effected faster than the direction of &lt;111&gt; by about 400 times. The solution of KOH is most safe and convenient for use but etches an oxidized film. In the case of etching for many hours, a nitride film is used as an etching protective mask.
FIG. 1 is a sectional view of a characteristic in the case where a silicon substrate crystallized in the direction of &lt;100&gt; is anisotropically etched. An oxide film or a nitride film is applied to the silicon substrate crystallized in the direction of &lt;100&gt; as an etching protective mask (102) and an etching window (103) is made only in a part to be etched (refer to FIG. 1(b)). When etching proceeds by means of an anisotropic etching solution like KOH and EDP, &lt;111&gt; surface (105) is exposed. In the &lt;111&gt; direction, etching is delayed as compared with &lt;100&gt; direction. Accordingly, as etching proceeds, a side &lt;111&gt; area increases and, when four sides meet at one point, it assumes the form of a pyramid (104) and etching is thereby finished (refer to FIG. 1(c)). At this time, the angle of side and substrate surface is 54.7.degree. and the maximum depth of etched range is determined by the size of the etching window (103).
FIG. 2 is a sectional view of a characteristic in the case where a silicon substrate crystallized in the direction of &lt;410&gt; is anisotropically etched. An oxide film or a nitride film is applied to the silicon substrate crystallized in the of &lt;410&gt; as an etching protective mask (202) and an etching window (203) is made only in a part to be etched (refer to (b) of FIG. 2). When etching proceeds by means of an anisotropic etching solution like KOH and EDP, &lt;111&gt; surface (205) is exposed. In the &lt;111&gt; direction, etching is delayed as compared with &lt;410&gt; direction. Consequently, as etching proceeds, &lt;111&gt; surface (205) is exposed on the side and etched area decreases. When four sides meet at one point, it assumes the form of a pyramid (204) and etching is thereby finished. (refer to (c) of FIG. 2). At this time, the angle of side and substrate surface is 45.6.degree. .
FIG. 3 is a sectional view of a characteristic in the case where a silicon substrate crystallized in the direction of &lt;110&gt; is anisotropically etched. An oxide film or a nitride film is applied to a silicon substrate (301) crystallized in the &lt;110&gt; direction as shown in (a) of FIG. 3 as an etching protective mask (302) and an etching window (303) is made only in a part to be etched (refer to (b) of FIG. 3).
When etching proceeds by means of an anisotropic etching solution like KOH and EDP, &lt;111&gt; surface (306) appears on the side at an angle of 90.degree. with the substrate surface and etching to the side is not effected any more. In the bottom (305) of etched part, etching continues to proceed to the &lt;110&gt; surface, and so it assumes the form of an etching square pillar (304) surrounded by four sides (306) in the &lt;111&gt; direction and etching is thereby finished (refer to (c) of FIG. 3).
Therefore, the maximum depth of an etched range is determined simply by etching time regardless of the size of the etching window (303).
Examining those papers which are related to the aforesaid anisotropic etching technique, there is mentioned optical waveguide manufacturing technique by the anisotropic etching and bonding of silicon substrates in pages 508-509, Electronic Letters 21 (1985), "silicon optical PCB for 3-D integrated optics". of Y. Kokubum. In pages 1321-1327, J. Electrochem. Soc. "The fabrication of high precision nozzle by the anisotropic etching of &lt;100&gt; silicon." of E. Bassous, there is mentioned technique of manufacturing ink jet nozzles by etching a &lt;100&gt; silicon substrate with a solution of EDP.
And, in "A micromachined biomedical pressure sensor with fiberotic interferometric readout", Proc. 7th Int'l Confon Solid-State Sensors and Actuators, there is mentioned "technique of etching structure having a side face at an angle of 45.degree. with the plane of a substrate.
According to those papers, however, in the case where microstructures assuming complicated forms on a silicon substrate are manufactured, a method of manufacturing each part severally on an individual substrate and then uniting said each part is used. As this method requires an individual photograph transferring process and etching process for every substrate, it is troublesome to perform the aforesaid processes as many times as the number of substrates and microstructure are apt to change their shapes or suffer a damage in the process of uniting the manufactured microstructure.
According to those papers, the shape of an ink jet nozzle comprises four simply shaped inclined planes making an angle of 54.degree. with the surface of a substrate and an optical reflective surface is also made of a surface which sank at an angle of 54 degrees or 45 degrees with the surface of the substrate.
As the substrate is anisotropically etched in such a simple way, an optical divider indispensable when a nozzle or a light signal is treated can not be manufactured in a fine and miniaturized form and the accuracy of a position is also lowered, not to mention that the structure is not properly applicable.
As a sensor provided with a sensing part which reacts mechanically and a peripheral circuit part which processes a signal from the sensing part on a signal chip by the use of a silicon substrate is developed recently, a growing interest is taken therein.
In the case of a pressure sensor, the sensitivity of a sensing part is proportional to the square of the length of one side of a diaphragm and inversely proportional to the square of thickness. Therefore, in the case where the aforesaid sensor is manufactured, it is important to manufacture a diaphragm which is thin and uniform in thickness and wide in area as far as possible.
FIG. 4 is a sectional view of a process in the case where a diaphragm is manufactured. A diaphragm (403) is manufactured (refer to b of FIG. 4) by forming an etching protective mask (402) on one surface of a substrate (401) like (a) of FIG. 4 and conducting etching therein.
FIG. 5 is a sectional view of a process in the case where a diaphragm is manufactured in bonded substrates crystallized in the same direction with an epitaxial layer interposed therein. In the first place, a p.sup.- substrate (503) is bonded to the lower part of a p.sup.+ substrate (501) with a p.sup.- epitaxial layer (502) formed (refer to (a)(b)(c) of FIG. 5). When only the p.sup.+ substrate (501) is etched by means of an etching solution which effects etching selectively according to the concentration of an impurity, for example, a mixed solution of fluoric acid-nitric acid-acetic acid, the p.sup.- epitaxial layer is left on the p.sup.- substrate (503).
In such a condition, a diaphragm (504) is manufactured by etching the p.sup.- substrate (503). The thickness of a thin film diaphragm in the process of FIG. 4 and FIG. 5 is determined simply by the etching time of a final substrate, and so it offers a problem in the uniformity and reproducibility of the thickness of the diaphragm.